A coupling function for solar wind effect on geomagnetic activity

Abstract

[1] From a simple theoretical consideration, we obtained two coupling functions linking upstream solar wind parameters to geomagnetic activity. We took into account (1) a scaling factor related to polar cap expansion while increasing the reconnected magnetic flux in the dayside magnetosphere, and (2) a modified Akasofu function for the reconnected flux for combined IMF B z and B y components. One of these coupling functions may be written as F a = aV sw B yz 1/2 sin a (q/2), where V sw is the solar wind speed, B yz is the magnitude of the IMF vector in the Y-Z plane, q is the clock angle between the Z axis and IMF vector in the Y-Z plane, a is a coefficient, and the exponent, a, is derived from the experimental data and equals approximately to 2. The F a function is proportional to the square root of B yz , which makes it significantly different from the coupling functions proposed earlier. Nevertheless, the statistical data analysis supports this dependence. For testing the found coupling function, we used the solar wind and IMF data for four years. We computed 2-D diagrams showing the correlation coefficients for the dependence of the polar cap PC geomagnetic activity index on different combinations of solar wind/IMF parameters. The obtained diagrams showed very good agreement with the theoretical coupling function. The correlation coefficient for the dependence of the PC index on the coupling function is about 0.8-0.85, which is significantly higher that that for other commonly used coupling functions for the same time intervals.

Alfven Wave Reflection model of field-aligned currents at Mercury

“…These values are close to those reported by Slavin et al (2009) who found the normal magnetic field to the magnetopause during the second MESSEN-GER flyby of $13 nT and the cross-magnetospheric electric field E 1 % 2 mV/m (see also Blomberg et al, 2006). Taking into account a significant contribution to the magnetospheric convection from IMF B y component (e.g., Lyatsky et al, 2007, the references therein), we obtain the average electric field in the Mercury's magnetosphere some higher, say E 1 % 5 mV/m. Due to convergence of the magnetic field lines, the electric field E 1 near the Mercury's surface may be two-three times stronger.…”

Validation of community models: 2. Development of a baseline using the Wang‐Sheeley‐Arge model

“…A detailed description of our approach in analyzing these events is presented by MacNeice [2009]. A number of studies [ Lyatsky et al , 2007; Perreault and Akasofu , 1978] have shown that the geomagnetic disturbance on the ground in the dayside polar cap region is well correlated with the product of the solar wind speed and the southward component of the interplanetary magnetic field (IMF). Neither the WSA model, or the heliospheric MHD models adequately forecast the southward component of the IMF.…”

A new polar magnetic index of geomagnetic activity

“…[7] For calculation of the polar magnetic (PM) index, we used the Akasofu function [Perreault and Akasofu, 1978;Akasofu, 1981; see also Lyatsky et al, 2007, and references therein]…”

“…Lyatsky et al [2007] proposed a coupling function, obtained from a simple theoretical consideration and linking upstream solar wind data to geomagnetic activity. They used the Perreault-Akasofu method [Perreault and Akasofu, 1978] and took into account a scaling factor due to polar cap expansion while increasing a reconnected magnetic flux in the dayside magnetosphere.…”